A permanent magnet motor consists of a wound stator within which a rotor rotates. Permanent magnets are attached to the rotor to produce alternating north and south magnetic fields that interact with electrical current through the stator to produce torque. The permanent magnets are attracted to the steel core of the rotor, but the centrifugal force created by the rotation of the rotor tends to pull the magnets off of the rotor.
Magnet retention is difficult, involving several factors. First, the magnets are brittle ceramics and structurally weak. Second, the centrifugal forces are high, especially with very high-speed rotors. Third, radial space (i.e., the space between the rotor and the stator) is at a premium because the magnetic field weakens as the radial separation between the rotor and stator increases. Fourth, permanent magnet motors frequently are required to operate in environments spanning a wide range of temperatures and the rates of thermal expansion of the components of the rotor may differ substantially over the temperature range.
Many methods have been proposed to retain magnets on rotors. Magnets can be bonded to the surface of the rotor, and then held in place by an outer wrap of high-strength material such as glass or carbon fiber, typically with an encapsulant filling the spaces between magnets. These methods have a drawback in that the thickness of the wrap reduces the mechanical clearance (i.e., the radial space) between the stator and rotor. Also, the expansion rate of the wrap under tension and temperature makes it difficult to keep the adhesive bond in compression at high rotational speed. In the absence of compression the adhesive bond can peel, which then allows the magnets to move axially. Finally, since these approaches depend on the integrity of the outer wrap, it is not feasible to repair or replace a magnet after the rotor has been built. The prior art teaches a similar method without the outer wrap. This approach eliminates the radial thickness penalty of the approaches described above, but relies totally on the encapsulant and bond for retention.
Another prior art approach teaches a detachable magnet carrier to hold the magnets. Essentially, the magnets are packaged in a stainless steel box that provides structural strength. This is an expensive approach, and the thickness of the box subtracts from the radial clearance between the rotor and stator.
Magnets can also be contained inside of the rotor, such that the rotor structure retains the magnets. Interior magnet constructions require compromises in the magnetic circuit that reduce performance in some applications.
Accordingly, there still exists a need in industry for a magnet mounting method and structure that places the magnets on the surface of the rotor, using a minimal radial thickness of structural material, so that the performance of the magnetic circuit is maximized. Further, the mounting should maintain compression on the magnets under a wide range of rotational speed and temperature, avoiding excessive mechanical stress on the brittle ceramic magnets. Finally, the mounting should allow the replacement of individual magnets after the rotor has been built.